In the downlink direction from 3rd generation radio access networks to terminal devices, the length of each transmitted block of data (called transport block) is not fixed but changes basically all the time. The length of a block of data is thus unknown to the receiver. The term “transport format” is used here as a synonym for the transport block length. The transport format contains also other pieces of information than the block length, but the transport format is detected by detecting the block length. The length of a particular transport block can be signaled in a control channel using a transport format combination indicator (TFCI). If the length is not signaled, then the receiver has to detect the block length blindly. That operation is called blind transport format detection (BTFD).
The 3GPP (3rd Generation Partnership Project) specifications specify three types of BTFD. The names and brief simplified descriptions of them are presented below:                1. An Explicit Blind Transport Format Detection (EBTFD), where the base station has transmitted a transport block including data and a cyclic redundancy code (CRC). The length of the data block is unknown, but a set of possible lengths is known. The receiver has to estimate which block length is the most likely one.        2. A Guided Transport Format Detection (GTFD) which basically doesn't require any additional computing. The result of the EBTFD is used for GTFD.        3. A Single Transport Format Detection (STFD), where the base station has transmitted either nothing at all, or a transport block (set) including data and CRC. In other words, the transport block can have only one length. It is transmitted with that length or not at all. The receiver has to estimate which alternative is more likely.        
In the following, two known approaches for blind transport format detection are described.
For STFD, where the possible data rates are zero and full rate, and CRC is only transmitted for full rate, blind transport format detection using received power ratio can be used. The transport format detection is then done using an average received power ratio of the Dedicated Physical Data Channel (DPDCH) to the Dedicated Physical Control Channel (DPCCH). In particular, the received power Pc per bit of the DPCCH is calculated from all pilot and power control bits per slot over a radio frame. Then, the received power Pd per bit of the DPDCH is calculated from X bits per slot over a radio frame, wherein X designates the number of DPDCH bits per slot when the transport format corresponds to full rate. If the average received power ratio Pd/Pc is determined to be larger than a threshold T for trans-port format detection, then the full rate transport format is detected. Else, the zero rate transport format detected.
For EBTFD, where the possible data rates are 0, . . . , (full rate)/r, . . . , full rate, and CRC is transmitted for all transport formats, blind transport format detection using CRC can be used. At the transmitter, the data stream with variable number of bits from higher layers is block-encoded using a cyclic redundancy check (CRC) and then convolutionally encoded. The CRC parity bits are attached just after the data stream with variable number of bits.
FIG. 2 shows an example of data with variable number of bits. In this example, four possible transport formats are available, and the transmitted end bit position has been selected as nend=3.
In a known procedure of blind transport format detection using CRC, the receiver knows only the possible transport formats (or the possible end bit position) based on Layer-3 (L3) negotiation. The receiver performs Viterbi-decoding on the soft decision sample sequence. The correct trellis path of the Viterbi-decoder ends at the zero state at the correct end bit position. The blind transport format detection method using CRC traces back the surviving trellis path ending at the zero state (hypothetical trellis path) at each possible end bit position to recover the data sequence. For each recovered data sequence error-detection is performed by checking the CRC, and if there is no error, the recovered sequence is declared to be correct.
The following variable is defined:S(nend)=−10 log((a0(nend)−amin(nend))/(amax(nend)−amin(nend)))[dB]  (1)where amax(nend) and amin(nend) are the maximum and minimum path-metric values among all survivors at end bit position nend, and a0(nend) is the path-metric value at zero state.
In general, the term “metric” is used hereinafter to designate a measure of similarity between a received code word or signal and one of allowed or candidate code words or signals defined by the underlying coding procedure.
In order to reduce the probability of false detection (which happens if the selected path is wrong but the CRC misses the error detection), a path selection threshold D is introduced. The threshold D determines whether the hypothetical trellis path connected to the zero state should be traced back or not at each end bit position nend. If the hypothetical trellis path connected to the zero state that satisfies the equation:S(nend)≦D  (2)is found, the path is traced back to recover the frame data, where D is the path selection threshold and a design parameter.
If more than one end bit positions satisfying equation (2) is found, the end bit position which has minimum value of S(nend) is declared to be correct. If no path satisfying equation (2) is found even after all possible end bit positions have been exhausted, the received frame data is declared to be in error.
However, the above known procedures are quite complex and require separate handling of the EBTFD and STFD.